High-frequency wavepackets in turbulent jets

Abstract

Wavepackets obtained as solutions of the flow equations linearised around the mean flow have been shown in recent work to yield good agreement with the amplitudes and phases of turbulent fluctuations in jets. Compelling agreement has been demonstrated up to Strouhal numbers, St ≈ 1. We extend the range of validity of wavepacket models to higher values, 1.0 < St < 4.0, by comparing Parabolised Stability Equation solutions with well resolved large-eddy simulation data. The initial growth rates of the high-frequency fluctuations continue to be well predicted, but saturation occurs earlier and agreement with simulation begins to deteriorate upstream of the end of the potential core of the jet. Results show that near-nozzle dynamics for a broad range of frequencies can be modelled using linearised models, which capture well the spatial growth of Kelvin-Helmholtz wavepackets for all the studied Strouhal numbers.